For those with deep pockets, here is the successor to the T223 (though with a lower serial number): 
www.rockislandauction.com
Tony
Vietnam Era Prototype H&K HK223, Class III/NFA C&R Machine Gun
Extremely Rare Vietnam Era Prototype Factory Original German Heckler & Koch HK223 Selective Fire Rifle, Class III/NFA C&R Fully Transferable Machine GunManufactured in 1966 as dated on the left of the magazine well. Offered here is an incredibly rare example of a registered fully transferable...
www.rockislandauction.com
Tony